1. Field of the Invention
The present invention relates to a metering servo valve for a fuel injector of an internal-combustion engine.
2. Description of the Related Art
As is known, the servo valve of an injector in general comprises a control chamber of the usual control rod of the injector nozzle. The control chamber is provided with an inlet hole in communication with a pipe for the pressurized fuel and a calibrated hole for outlet or discharge of the fuel, which is normally closed by an open/close element. Normally, the valve body of the servo valve is fixed on a shell of the injector, whilst the open/close element is controlled by the armature of an electromagnet.
The travel or lift of the armature determines the readiness of the response of the servo valve both for opening and for closing, as well as the section of passage of the fuel through the discharge hole, so that it is necessary to regulate accurately the travel of the armature and/or of the open/close element. Servo valves are known with the open/close element separate from the armature, the travel of which is defined on the one hand by arrest against the open/close element in a position of closing of the discharge hole and on the other by arrest of the travel of the armature in the direction of the electromagnet. Adjustment of the travel of the armature is made using at least one rigid shim, which defines the gap of the armature. The shim can be chosen from among classes of calibrated and modular shims. For technological reasons and for economic constraints of feasibility, said shims can vary from one another by an amount of not less than the machining tolerance, for example, 5 μm. The operation of adjustment of the travel of the armature by discrete amounts with a tolerance of 5 μm is, however, relatively rough so that it is often impossible to obtain a flow rate of the injector within the very narrow limits required by modern internal-combustion engines.
From the document EP-A-0 916 843, a servo valve is also known, in which the armature is guided by a sleeve, which carries the arrest element of the armature in the direction of the electromagnet. The sleeve is moreover provided with a flange, which is fixed on the shell, with the interposition of an elastically deformable shim. The electromagnet is housed in a casing, which is fixed on the shell of the injector by means of a threaded ring nut and is provided with a portion acting on the aforesaid flange. The shim is deformed according to the tightening torque of the ring nut so that, by varying said torque, a fine adjustment of the travel of the armature is obtained. However, the presence of said shim and the corresponding selection render the servo valve relatively complicated and costly to manufacture.
In addition, in the known servo valve described above, the open/close element is subjected on one side to the axial thrust exerted by the pressure of the fuel in the control chamber, and on the other to the action of axial thrust of a spring, which is pre-loaded so as to overcome the thrust of the pressure when the electromagnet is not excited. The spring has hence characteristics and overall dimensions such as to be able to exert a considerable axial thrust, for example, in the region of 70 N for a fuel pressure of 1800 bar.
In order to reduce pre-loading of the spring for closing the open/close element, a servo valve has recently been proposed, in which the pressurized fuel no longer exerts an axial action, but acts in a radial direction on the support of the open/close element so that the action of the pressure of the fuel on the open/close element is substantially balanced. The action of the spring and that of the electromagnet can hence be reduced. In addition, the travel of the armature can stop directly against the core of the electromagnet, given that the risk of sticking of the armature is negligible, so that the residual gap with respect to the core itself can be eliminated.